BACKGROUND: The object of this study is to offer the optimal angle of needle insertion during L3-L4 paramedian approach using simple X-ray.

METHODS: Twenty-five male patients were enrolled for this study. After placing a radio-opaque material (RO) on the point 1cm lateral and 1cm caudad to inferior edge of L3 spinous process in the sitting-flexion position, simple AP X-ray films in erect position and lateral films in sitting-flexion position were taken. The distance from RO (I) to the midline of vertebral column was measured on the AP film. On the lateral film, the optimal target point (T) was determined. When the real RO (I) appeared on lateral film, the point was named I'. The line perpendicular to the line tangential to skin on I' was drawn to the vertebral body. When another line perpendicular to that line was drawn from the target point (T), the two lines meet perpendicularly at the point C. Two triangles can be formed three-dimensionally with T, I', C and T, I, C. Medial insertion angle (α, angle I-C-I') and cephalad insertion angle (β, angle T-I-C) were calculated.

RESULTS: The mean angle of α was 10.7 ± 2.3° and β was 13.9 ± 5.0°. The insertion based on the calculated angles was successfully achieved at the first trial in 24 patients and at the second in 1 patient.

CONCLUSIONS: Spinal anesthesia with L3-L4 paramedian approach can be successfully performed using calculated angles measured by simple X-rays.